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WORLD PUMPS April 2013

Variable-speed drives (VSDs) provide

the user with a variety of benefi ts,

including potentially signifi cant

energy savings and improved reliability in

pumping applications. Assessments of the

technical and economic advantages

gained by using VSDs on circulation

pumps have been widely publicized in

recent years. According to statistics,

pumps consume around 20% of the

world’s total electrical energy1. The energy

effi ciency of a system depends not only

on the design of the pump but also, and

more so, on its working conditions and

system design.

An optimal control method for

variable-speed pumping stations with

similar pumps in parallel has

been proposed by Fulai and Hexu2.

This control method gives the

minimum total power consumption for

similar variable-speed pumps running in

parallel.

A number of researchers and experts in

the heating, ventilation and air-condi-

tioning (HVAC) fi eld have also devoted

considerable eff orts to applying proper

and optimal control strategies to enhance

the effi ciencies of circulation pumps.

Optimal control strategies to enhance the

energy effi ciencies of variable-speed

pumps with diff erent confi gurations in

complex building air-conditioning systems

have been presented3. Ahonen et al.4

have proposed two model-based

methods for the estimation of pump

taking into account their power

consumptions.

Circulation pumps

Circulation pumps are among the largest

consumers of electricity in private houses

and commercial buildings, and are widely

used in heating and cooling systems.

They are generally in-line-type pumps and

are typically used to recirculate heating or

cooling media within a closed loop

(Figure 1). They only need to overcome

the friction losses of the piping system.

Speed variation is the most energy-effi -

cient method to respond to variable

system demands4. Frequency converters

are widely used nowadays to control the

head and fl ow rate of circulation pumps.

Traditionally, a circulation pump was

driven by a constant-speed induction

motor with system demand controlled by

operation. One of the methods is based

on the fl ow rate and head of the pump

and the other method is based on the

system curve. Tianyi et al.5 recently

proposed an on-line control optimization

method for variable fl ow pumps in

air-conditioning systems. This method

includes the optimized allocation of

parallel, variable-frequency pump speed

ratios and the optimized number of

operating pumps.

The main purpose of the study described

in this article is to determine the optimal

control strategy for similar variable-speed

circulation pumps under parallel running

conditions. The control strategy is based

on the optimal combination of speed

ratios of multiple parallel variable speed

pumps that utilize the same hydraulic

operating conditions. In addition, the

control strategy determines the optimal

number of pumps in parallel operation

Circulation pumps are widely used in HVAC systems but are heavy

consumers of electricity. Here, M. Karaca and M. Aydin consider

strategies to optimize the energy use in a closed-loop system of

parallel, variable-speed circulation pumps, and demonstrate the

potential of the speed ratio to provide large energy savings.

Effi cient driving at variable speeds

HVAC

0262 1762/13 © 2013 Elsevier Ltd. All rights reserved

Figure 1. Simplifi ed diagram of a typical heating system with a circulation pump.

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a throttling valve. However, system

demand can now be regulated through

frequency converters without the

additional losses caused by throttling.

By introducing more-effi cient pumps and

control strategies, it is possible to reduce

electricity consumption considerably, as

shown in Figure 2. In this fi gure, realistic

policy measures (Scenario 1; Sc1) applied

to circulation pumps are projected to

reduce electricity consumption in the

European Union (EU) dramatically6

compared to maintenance of the status

quo as indicated by the reference curve

(RC). Here, ETP refers to economically

justifi able and technically feasible

potential energy savings by circulation

pumps.

The potential reductions in electricity

usage would positively aff ect consumers’

electricity bills as well as carbon dioxide

emissions. Depending on the emission

factor used, carbon dioxide emissions

could be reduced by in excess of

5 million tonnes per year by 2020, as

indicated in Table 1 from the diff erence

between projected RC and Sc1 outputs.

Mathematical model

A mathematical model to characterize

pump performance is given as follows:

H = a1Q2 + a2Q + a3 (1)

η = b1Q2 + b2Q + b3 (2)

where H (m) is the head of the pump at

the rated speed; Q (m3/s) is the fl ow of

the pump at rated speed; and a1, a2 and

a3 are constants obtained from the

sample pump performance curve. In

Equation 2, η (%) is the pump effi ciency

at the rated speed; and b1, b2 and b3 are

further constants obtained from the

pump performance curve.

The power, head, fl ow and effi ciency

produced by the pump are defi ned as

follows:

P = FP 3

w Q =

w

Q F H =

FH 2

w η = ηF (3)

where P (kW) is the power of the pump at

the rated speed; w is the speed ratio, that

is, the ratio of the variable speed to the

rated speed; and the subscript F denotes

operation at variable speed. The power at

the rated speed (P) and at variable speed

(PF) can be expressed as follows:

P = 10QH

Mη

VFDη η

(4)

PF = F

ηM

ηVFD

ηFQ10 FH

(5)

where ηM (%) is the effi ciency of the

motor and ηVFD (%) the effi ciency of the

frequency converter.

According to the mathematical model, for

any two or more identical pumps running

in parallel that correspond to the same

hydraulic conditions (i.e. operating at the

same temperature and pressure), the best

overall effi ciency can be achieved with

the same speed ratio. The mathematical

model was explained in detail by Tianyi et al5.

Normally, changing the speed does not

aff ect the pump effi ciency. However, when a

pump with a frequency converter is driven

at a lower speed than the rated speed, the

motor effi ciency and the frequency

converter effi ciency will change due to the

loading. Because of that, the system effi -

ciency (ηF + ηM + ηVFD) can be changed at

lower speeds than the rated speed (Figure 3).

Experimental setup

Tests were conducted to analyse the

optimal control strategy when a pump

operating in parallel is driven in diff erent

operating point confi gurations. Diff erent

speed ratios were applied to utilize the

same hydraulic conditions.

The equipment used in the tests is listed

below:

Circulation pump: in-line type 40-160

pump

Induction motor: Wat 1.1 kW, 1,500 rpm

Frequency converter: ABB ACS310 2.2 kW

Pressure transmitter: Keller PA-21 Y/4 bar

(0–4 bar, 4–20 mA)

Electromagnetic fl ow meter: Khrone

IFC100 DN50

Multimeter: Entes EPM-07

Figure 2. Electricity consumption by circulation pumps in the EU according to current (RC) and potential

consumption scenarios6 (reproduced courtesy of Grundfos A/S).

Table 1. Summary of EU electricity consumption scenarios and

potential savings for circulation pumps

Electricity

(TWh)

Electricity

costs

(million Euros)

CO2

emissions

(Mt CO2)

RC, 2000 37.6 3,756 15.1

RC, 2020 45.6 4,558 18.4

ETP, 2020 13.9 1,394 5.6

Scenario 1 (Sc1), 2020 31.7 3,173 12.8

(RC–Sc1), 2020 13.9 1,385 5.6

From Ref. 6 (reproduced courtesy of Grundfos A/S)

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.

.

.

.

.

.

.

.

points were evaluated in the tests to

demonstrate the energy saving potentials

associated with the use of the optimal

control method for variable-speed circula-

tion pumps. For each duty point, the

supplied power (Psup) – which is the basic

factor for the energy consumption of the

entire pump unit – was obtained by

means of a multimeter changing the

speed of the pump, which otherwise

operates under the same hydraulic

conditions during the test. Table 2

presents the supplied powers for the

diff erent operating conditions and for

various confi gurations of the test pumps.

Based on the test results as shown in

Table 2, the pump speed ratio signifi -

cantly aff ects the energy saving. In addi-

tion to that, the overall best effi ciency is

obtained when pumps have the same

speed ratios. In the variable-speed condi-

tion, with the same speed ratio and the

minimum number of pumps, power

savings of 16.9% for duty point 1, 4.3% for

duty point 2 and 17.3% for duty point 3

were achieved, according to the test

results. Therefore, the main criteria for

optimal system control are to use the

minimum number of pumps to meet the

system demand and the same speed

ratio; Psup was a minimum for these

conditions.

The experimental results show that some

70% of the pump energy could be saved

by using this optimal control strategy

when compared to traditional control

methods. For instance, consider duty Figure 3. The reference single pump performance curve for the variable-speed circulation pump used in the study.

Figure 4. The closed-loop test system.

The experimental test system is shown in

Figure 4; all equipment was calibrated

before carrying out the measurements. As

shown, three in-line circulation pumps of

the same type were connected to a

common collector.

The head was measured with pressure

sensors placed on the suction and

discharge fl anges of the circulation

pumps; the fl ow rate is measured by the

electromagnetic fl ow meter. Frequency

converters are able to control the pump

according to the pressure diff erence

between pump outlet and inlet. It is

worth noting that a constant diff erential

pressure algorithm was generated for the

measurements of the operating points.

Diff erent operating points were generated

using the frequency converter.

The sample fl ow demand curve (Figure 5)

was generated for circulation pumps

running continuously for 24 hours to

analyse the control strategy. The nominal

fl ow demand, indicated on the graph by

the blue dashed line, was 19.7 m3/h. The

relative fl ow rates at the four selected

operating (duty) points were 14.7 m3/h,

19.7 m3/h, 24.6 m3/h and 25.3 m3/h. The

system’s static pressure was constant at

2 bar during the experiments.

Results and discussion

In pumping applications, the exact

amount of saved energy depends on the

system characteristic. This article focuses

on the application of parallel

variable-speed circulation pumps in a

closed-loop system. Four diff erent duty

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WORLD PUMPS April 2013

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.

.

.

cycle assessment would also be needed

to reach a conclusion about the future

usefulness of this control system.

Acknowledgement

The authors would like to thank Standart

Pompa R&D and Electrical Department

and Professor Haluk Karadoğan for their

generous support of this work.

References

[1] D. Kaya et al., ‘Energy effi ciency in

pumps’, Energy Conversion and

Management, Vol. 49, pp. 1662–1673,

(2008).

[2] Y. Fulai and S. Hexu, ‘Optimal control in

variable-speed pumping stations’,

Mechatronics and Automation (ICMA),

Vol. 28, pp. 2397–2401, (2011).

[3] M. Zhenjun and W. Shengwei, ‘Energy

effi cient control of variable speed

pumps in complex building central air

conditioning systems’, Energy and Build-

ings, Vol. 41, pp. 197–205, (2009).

[4] T. Ahonen et al., ‘Estimation of pump

operational state with model-based

methods’, Energy Conversion and

Management, Vol. 51, pp. 1319–1325,

(2010).

[5] Z. Tianyi, Z. Jili and M. Liangdong,

‘On-line optimization control method

based on extreme value analysis for

parallel variable-frequency hydraulic

pumps in central air-conditioning

systems’, Building and Environment, Vol.

47, pp. 330–338, (2012).

[6] N. Bidstrup, M. van Elburg and K. Lane,

‘Promotion of energy effi ciency in

circulation pumps, especially in

domestic heating systems’, EU Save II

Project Final Report, (2001).

[http://www.eci.ox.ac.uk/research/

energy/downloads/eusave-pump-

sumry.pdf ]

point 3, where the fl ow rate (Figure 5)

is 14.7 m3/h. Referring back to Figure 3,

the power for one pump running at 50

Hz power at this fl ow rate is 0.66 kW. For

the traditional control method,

involving the three test pumps running at

50 Hz, the total supplied power would

thus be:

0.66 × 3 =1.98 kW

If this fi gure for the traditional control

method is compared with the optimum

result determined for duty point 3 in

Table 2 – namely, one pump running at

42.7 Hz with a power usage of 0.67 kW –

this gives:

Power savings = (1.98–0.67)/1.98 = 0.66

The potential power savings for this duty

point is therefore 66%.

Conclusions

This study is based on the energy-effi cient

system operation of parallel-running, vari-

able-speed circulation pumps in a closed-

loop system. The results of this

investigation indicate that the best energy

saving can be obtained when the same

type of pumps, running in parallel, have

the same speed ratio. In addition, the

number of pumps running in parallel also

aff ects the energy-saving potential. The

test results show that the main criteria for

optimal system control are the minimum

number of pumps to meet the system

demand and the same speed ratio. Life

Contact

Metehan Karaca

Standart Pompa ve Mak. San. Tic. AS

Dudullu Organize San. Bol. 2. Cad. No. 9

Umraniye, İstanbul, Turkey

Email: metehan@standartpompa.com.tr

Murat Aydin

Istanbul Technical University (ITU)

Energy Institute, Ayazaga Campus

Maslak, İstanbul, Turkey

Email: aydinmurat@itu.edu.tr

Table 2. Experimental results

Duty point 1: H = 7 m Psup (kW)

2 pumps working at 37.1 Hz and 1 pump at 50 Hz 1.54

2 pumps at 42.7 Hz 1.15

3 pumps at 39.4 Hz 1.3

1 pump at 38.1 Hz and 1 pump at 50 Hz 1.28

Duty point 2: H = 10.3 m Psup (kW)

1 pump at 46.4 Hz and 2 pumps at 50 Hz 2.08

2 pumps at 47.5 Hz and 1 pump at 50 Hz 2.02

3 pumps at 48 Hz 1.99

Duty point 3: H = 3.7 m Psup (kW)

1 pump at 42.7 Hz 0.67

2 pumps at 33 Hz 0.68

3 pumps at 30.4 Hz 0.81

Duty point 4: H = 10.7 m Psup (kW)

3 pumps at 50 Hz 2.21

Figure 5. The sample fl ow demand curve.

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